Heretofore, for manufacturing a lithographic printing plate by an electrophotographic system, a lithographic printing plate precursor comprising a water resistant support having provided thereon a layer containing zinc oxide and a binder is subjected to corona electrical charging, imagewise exposed, toner-developed, fixed and then etched.
Examples of the water resistant support include waterproofed paper, waterproofed metal foil and a composite material thereof.
When paper is used as the support, in order to impart electrical conductivity to the paper, a coating solution containing an inorganic electrolyte such as sodium chloride, potassium chloride or calcium chloride, or an organic polyelectrolyte such as quaternary ammonium, which are called an electrically conducting agent, is used and the paper is dipped therein or coated with the solution. At this time, the volume resistivity of the paper is adjusted to about 1.times.10.sup.9 .OMEGA..cm.
However, when a lithographic printing plate precursor is manufactured using paper subjected to electrical conductivity treatment as the base material, due to the fountain solution applied during the printing, the paper on a roll at the printing cannot evade partial elongation, namely, plate elongation. As a result, wrinkles may be generated on the plate during the printing or the printed matter may be out of register to cause troubles such as geometric distortion of ruled lines.
In order to prevent the adverse effect of the fountain solution, JP-A-58-57994 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-59-64395 describe a structure where a laminate layer such as polyethylene containing an electrically conductive filler is provided. That is, use of an electrically conductive laminated paper has been attempted.
This laminated paper is, however, disadvantageous in that the paper support or resin film must be treated to have electrical conductivity, the production cost for the support increases and in turn, the cost of the printing plate as a whole increases.
Furthermore, for example, JP-B-38-17249 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-41-2426 and JP-B-41-12432 describe an attempt to use a paper having laminated thereon a metal foil such as aluminum, zinc or copper (hereinafter referred to as a "metal foil-laminated paper"). For the paper to be laminated in this case, the above-described paper impregnated with an electrically conducting agent is also used.
When this metal foil-laminated paper is used, the paper is necessary to be treated to have electrical conductivity and moreover, a metal foil must be laminated on one or both surfaces of the paper. Thus, the production cost is disadvantageously higher than that for the above-described laminated paper.
In this case, it may be considered to use a support obtained by forming an electrically conductive layer such as a metal foil on a normal polyester base or polyethylene-laminated paper base and further forming thereon a photoconductive layer. Such a support is inexpensive but on the other hand, the support as a whole is low in the electrical conductivity and cannot be used in practice. This is more specifically described below.
In the electrophotographic system, the lithographic printing plate is usually manufactured by the plate-making process where, as shown in FIG. 4, corona electrical charge is applied on both surfaces of a plate precursor In FIG. 4, a master 1' is passed through a negative corona charger 12 and a positive corona charger 19 before entering the exposure part 20, so that the upper part and the lower part of the photoconductive layer are charged to - and +, respectively, and then imagewise exposed at the exposure part 20. As a result, the electric charge in the exposed area is lost due to conduction of the photoconductive layer and the electric charge remains only in the non-exposed area to form an electrostatic image.
According to the print-making process having a construction as shown in FIG. 4, however, if the support is low in the electrical conductivity, the discharge phenomenon does not successfully occur and the image is deteriorated. It may be considered to directly contacting an electric conductor with the electrical conductive layer to earth and charge it, but since the lithographic printing plate is not repeatedly used and a new plate is always used, in view of the mechanism, an electric conductor is difficult to be contacted with the electrically conductive layer between the support and the photoconductive layer.
In the print-making process shown in FIG. 4, the exposure light irradiated from a light source is converged by a lens 18 at the exposure part 20. The exposure light converged forms an image on the master 1' which is fed from the paper feeding part 11 by a transportation means, subjected to electrostatic charging treatment and present in the exposure area 20 between guide rollers 15 and 16, thereby performing imagewise exposure or the master 1'. The exposed master 1' is transported to the development/fixing part 17 by the transportation means, developed by attaching toner to the non-exposed area, fixed, degreased, and dried to produce a lithographic printing plate.